The present invention relates to a method of monitoring refrigerant level (filling amount of refrigerant) in a refrigerant circuit of an air-conditioning or heat-pump system with a compressor and a refrigerant operated in the supercritical range as a function of the operating point.
In this case, a “refrigerant which may, depending on the operating point, be operated in the supercritical range” is to be understood as one which, at least for some of the possible system operating states, is in the supercritical range and is therefore present in vapor form even in the section of the refrigerant circuit from the “condenser”, (functioning at such time as a gas cooler) to the evaporator. In particular, the method is suitable for monitoring the CO2 level in CO2 air-conditioning systems, which are increasingly being used in motor vehicles. The method includes standstill level monitoring, that is to say level monitoring with the compressor switched off, and/or in-operation level monitoring, that is to say level monitoring with the compressor switched on.
U.S. Pat. No. 4,745,765 includes in-operation level monitoring in which conclusions are drawn about possible underfilling, by using the measured temperature of superheated refrigerant at the evaporator output and the ambient temperature.
U.S. Pat. No. 5,481,884 describes a method which includes both standstill and in-operation level monitoring. To this end, when the air-conditioning or heat-pump system considered there is at a standstill, the refrigerant pressure on the suction side of the compressor and the ambient temperature are measured. The saturation pressure associated with the measured ambient temperature is determined and used as a reference pressure, with which the measured refrigerant pressure is compared. If the measured pressure compared with the reference pressure is too low, it is concluded that there is underfilling. During operation of the system, temperature and pressure of the refrigerant are measured on the suction side of the compressor. The saturation temperature associated with the measured pressure is determined and used as a reference temperature, with which the measured refrigerant temperature is compared. If the measured temperature lies too far above the reference temperature, it is concluded that there is underfilling.